This invention relates generally to simplified feedthrough terminal pin subassemblies and related methods of construction, particularly of the type used in implantable medical devices such as cardiac pacemakers and the like, to decouple and shield undesirable electromagnetic interference (EMI) signals from the device. More specifically, this invention relates to a reduced cost and reduced mechanical stress hermetic feedthrough terminal pin and ceramic feedthrough capacitor assembly including one or more filter capacitors, and related installation method. It is adapted particularly for use in connecting a lead wire or electrode through a hermetically sealed housing to internal electronic components of the medical device while decoupling EMI against entry into the sealed housing. This invention is particularly designed for use in cardiac pacemakers (bradycardia devices), cardioverter defibrillators (tachycardia devices) and combined pacemaker defibrillator devices. This invention is also applicable to a wide range of other EMI filter applications, such as military or space electronic modules where it is desirable to preclude the entry of EMI into a sealed housing containing sensitive electronic circuitry.
Feedthrough terminal pin assemblies are generally well known in the art for connecting electrical signals through the housing or case of an electronic instrument. For example, in implantable medical devices such as cardiac pacemakers, defibrillators or the like, the terminal pin assembly comprises one or more conductive terminal pins supported by an insulator structure for feedthrough passage from the exterior to the interior of the medical device. Many different insulator structures and related mounting methods are known in the art for use in medical devices wherein the insulator structure also provides a hermetic seal to prevent entry of body fluids into the housing of the medical device. However, the feedthrough terminal pins are typically connected to one or more lead wires which effectively act as an antenna and thus tend to collect stray EMI signals for transmission into the interior of the medical device. In many prior art devices, the hermetic terminal pin assembly has been combined directly with a ceramic feedthrough filter capacitor to decouple interference signals to the housing of the medical device. A major market force within the medical implantable device industry has been to reduce cost of the implanted device (e.g. pacemaker or implantable cardioverter defibrillator). Medical insurance carriers, government healthcare programs (e.g. Medicare) and health maintenance organizations (HMOs) are placing additional competitive pressures on the manufacturers of such devices.
In a typical unipolar construction, as described in U.S. Pat. No. 5,333,095 (the contents of which are incorporated herein), a coaxial ceramic feedthrough filter capacitor used in a feedthrough assembly to suppress and decouple undesired interference or noise transmission along a terminal pin comprises a so-called discoidal capacitor having two sets of electrode plates embedded in spaced relation within an insulative dielectric substrate or base, formed typically as a ceramic monolithic structure. One set of the electrode plates is electrically connected at an inner diameter cylindrical surface of the discoidal capacitor structure to the conductive terminal pin utilized to pass the desired electrical signal or signals. The other or second set of electrode plates is coupled at an outer diameter surface of the discoidal capacitor to a cylindrical ferrule of conductive material, wherein the ferrule is electrically connected in turn to the conductive housing of the electronic device. The number and dielectric thickness spacing of the electrode plate sets varies in accordance with the capacitance value and the voltage rating of the discoidal capacitor. In operation, the discoidal capacitor permits passage of relatively low frequency electrical signals along the terminal pin, while shielding and decoupling/attenuating undesired interference signals of typically high frequency to the conductive housing. Feedthrough capacitors of this general type are available in unipolar (one,) bipolar (two), tripolar (three), quadpolar (four), pentapolar (five), hexpolar (six) and additional lead configurations. The feedthrough capacitors of this general type are commonly employed in implantable cardiac pacemakers and defibrillators and the like, wherein the pacemaker housing is constructed from a biocompatible metal, such as titanium alloy, which is electrically coupled to the feedthrough filter capacitor. As a result, the filter capacitor and terminal pin assembly prevents entrance of interference signals to the interior of the pacemaker housing, wherein such interference signals could otherwise adversely affect the desired cardiac pacing or defibrillation function.
In the past, feedthrough filter capacitors for cardiac pacemakers and the like have typically been constructed by preassembly of the discoidal capacitor within a cylindrical terminal pin subassembly which includes the conductive pin and ferrule. More specifically, the terminal pin subassembly is prefabricated to include one or more conductive terminal pins supported within the conductive ferrule by means of a hermetically sealed insulator ring or bead. See for example, the subassemblies disclosed in U.S. Pat. Nos. 3,920,888; 4,152,540; 4,421,947; and 4,424,551. An improved design which has substantially improved the volumetric efficiency is based upon surface mounting of a ceramic feedthrough capacitor planar array structure to one outer surface of a hermetic terminal with similar connection to the conductive pins (see the subassemblies disclosed in U.S. Pat. No. 5,333,095). In all of the prior art described above, the outer feedthrough capacitor electrode plate sets are coupled in parallel together by a metallized layer which is either fired, sputtered or plated onto the ceramic capacitor. This metallized band, in turn, is coupled to the ferrule by conductive adhesive, soldering, brazing, or the like.
Although feedthrough filter capacitor assemblies of the type described above have performed in a generally satisfactory manner, the manufacture and installation of such filter capacitor assemblies has been relatively time consuming and therefore costly. For example, installation of the discoidal capacitor into the small annular space described by U.S. Pat. No. 4,424,551 between the terminal pin and ferrule can be a difficult and complex multi-step procedure to ensure formation of reliable, high quality electrical connections. The method taught by U.S. Pat. No. 4,424,551 (the contents of which are incorporated herein), teaches the injection of fluidic thermosetting conductive particles into first and second annular cavities (usually by centrifuge operations). As a consequence, this method also requires insulation of the interface between the capacitor structure and insulator, curing of the various thermosetting materials, and subsequent cleaning operations to remove excess conductive material. While the method taught by U.S. Pat. No. 5,333,095 is far simpler, a connection from the capacitor outside diameter and the conductive ferrule is still required.
A high integrity hermetic seal for medical implant applications is very critical to prevent the ingress of body fluids into the implanted device (e.g. pacemaker). Even a small leak rate of such body fluid penetration can, over a period of many years, build up and damage sensitive internal electronic components. This can cause catastrophic failure of the implanted device. The hermetic seal for medical implant (as well as space and military) applications is typically constructed of highly stable alumina ceramic or glass materials with very low bulk permeability. A helium fine leak test is typically used in conjunction with a sensitive detector to reject defective or cracked hermetic seals. This final product quality conformance test is typically of very short duration (a few seconds helium exposure). This short test exposure will readily detect a leak in a cracked or otherwise defective alumina ceramic or glass hermetic seal; however, it typically takes much longer for helium to penetrate through an epoxy or polyimide adjunct barrier (such polymer overcoating can mask the leak).
Withstanding the high temperature and thermal stresses associated with the welding of a hermetically sealed terminal with a premounted ceramic feedthrough capacitor is very difficult to achieve with the '551, '095 and other prior art designs. The electrical/mechanical connection to the outside perimeter or outside diameter of the feedthrough capacitor has a very high thermal conductivity as compared to air. The welding operation typically employed in the medical implant industry to install the filtered hermetic terminal into the ICD can involve a welding operation in very close proximity to this electrical/mechanical connection area. Accordingly, in the prior art the ceramic feedthrough capacitor is subjected to a dramatic temperature rise. This temperature rise produces mechanical stress in the capacitor due to the mismatch in thermal coefficients of expansion of the surrounding materials. In addition, in the prior art the capacitor lead connections must be of very high temperature materials to withstand the high peak temperatures reached during the welding operation (as much as 500.degree. C.). A similar, but less severe, situation is applicable in military, space and commercial applications where similar prior art devices are soldered instead of welded by the user into a bulkhead or substrate. Many of these prior art devices employ a soldered connection to the outside perimeter or outside diameter of the feedthrough capacitor. Excessive installation soldering heat has been known to damage such prior art devices.
Accordingly, there is a need for a novel feedthrough filter capacitor assembly that addresses the drawbacks noted above in connection with the prior art. In particular, a novel capacitor assembly is needed that is subjected to far less temperature rise during the manufacture thereof by eliminating an outside perimeter or outside diameter electrical/mechanical connection. Such a design would allow the use of much lower temperature materials (such as standard solder) to achieve the capacitor inside diameter lead connections. Moreover, such an improvement would make the assembly relatively immune to the aforementioned stressful installation techniques. Moreover, a novel filter capacitor design is needed which is of simplified construction, utilizing a straightforward and uncomplicated feedthrough terminal pin subassembly, that can result in manufacturing cost reductions. Of course the new design must be capable of effectively filtering out undesirable electromagnetic interference (EMI) signals from the target device. The present invention fulfills these needs and provides other related advantages.